1. Field of the Invention
The present invention relates to an improved method of configuring and using a furnace. More specifically, the invention relates to an improved method to reduce deflections of burner flames in down-fired, steam methane reforming furnaces in order to optimize performance.
2. Prior Art
Steam methane reforming (“SMR”) processes for the production of synthesis gas are well known in the art. SMR involves a reaction of a hydrocarbon feedstock, typically in the form of natural gas, refinery gas, or naphtha, with steam at high temperatures. This reaction is in the presence of catalysts and produces a gas mixture primarily made up of hydrogen and carbon monoxide, commonly known as syngas. The syngas is collected and further processed in another system.
The SMR reaction may take place within different types of furnace configurations with burners on the top, sides, or bottom of the furnace. A typical configuration includes multiple cell or chamber furnaces with rows of down-fired burners lining each cell or chamber. Each cell is separated by vertically aligned rows of catalyst-filled process tubes.
In such a furnace, there may be several center cells and two outer cells. The outer most cells are adjacent to a refractory lined furnace wall. Therefore, they only have process tubes on one side of the cell. The center cells may vary in number from one (1) to several, depending on the furnace capacity. Each center cell is bounded by a row of process tubes along either side. The top of each cell is lined with multiple, down-fired burners.
The hydrocarbons and the steam are introduced at the top of the process tubes thereby to react with the catalyst contained within the process tubes in order to form syngas (primarily H2 and CO). The syngas is removed through the bottom of the process tubes and further refined by another process. Forming syngas from hydrocarbons and steam is a highly endothermic reaction requiring much heat. The rows of down-fired burners generate flames to provide the heat necessary for the reaction. Below such flames, furnace gases and combustion products flow downward and finally out through inlets of tunnels located in each cell.
In a typical down-fired burner furnace, the center cells have process tubes on either side, therefore there is twice as much heat transfer surface in a center cell than there is in an outer cell, which is lined with process tubes on one side and then refractory lined furnace wall on the other side. Further, the outer cells normally have fifty percent to sixty-five percent (50%-65%) of the firing rate when compared to the center cells and as a result, the outer cells only have fifty percent to sixty-five percent (50%-65%) of the mass flow rate of the center cell burners.
Multiple chambered, down-fired reforming furnaces have complex gas flow patterns. Open space between each individual process tube allows for combustion products from the down-fired burners to pass into the adjacent cell, and vice versa. The combustion product flow patterns can be strong enough to cause deflections of the burner flames which ultimately leads to uneven heating of the process tubes and excessive tube metal temperatures. This combustion product flow between adjacent cells is a result of unequal flow momentum from the mass flow rate discrepancy between the outer cells and the center cells.
The larger, center burners and the smaller, outer burners are both designed for the same air pressure loss. Assuming the burners are geometrically proportional to the flow, then the velocity of the gases exiting the burner tile will be the same for both sized burners. The gases exit through a burner discharge area at a certain velocity termed a burner discharge velocity. However, the mass flow rate of the outer cell burners will be fifty percent to sixty-five percent (50%-65%) of the mass flow rate of the center cell burners. As a result, the momentum of the combustion product flow from the outer cell burners will also be fifty percent to sixty-five percent (50%-65%) of the center cell burners. The higher momentum of the center cell burners creates a lower static pressure zone at the burner discharge area near the top of the center cells than is present at the top of the outer cells. This static pressure difference causes the combustion products from the outer cells to flow toward the center cells. The combustion product flow also causes the flames from the outer cell burners to bend toward the process tubes of the center cells and in many cases, the flames cause the metal of the process tubes to overheat. Reducing the temperature of the process tubes would require reducing the firing rate of the burners which would ultimately limit the capacity of the furnace.
The foregoing gas flow problems are known in the art. Joshi et al., U.S. Pat. No. 7,686,611 B2 discloses a method and apparatus for generating straightened flames in a furnace. The method and corresponding apparatus involves the addition of oxidant conduits to introduce an oxidant to the fuel.
DiMartino, Sr., et al. U.S. Pat. No. 5,795,148 also attempts to cure the problem of unstable flame patterns and finds that wind external to the furnace is a contributing factor to the problem. DiMartino teaches an apparatus that controls the amount of air received by the furnace burners and provides uniform air pressure going to the burners.
However, neither Joshi et al., nor DiMartino, Sr. et al., nor the other prior art account for the mass flow discrepancy between the center cell burners and the outer cell burners.
It is therefore desirable to provide a method for use in a furnace which provides for substantially uniform temperatures between the multiple rows of process tubes, without the process tubes overheating.
It is further desirable to provide a method for use in a furnace which provides for the combustion product flow to have a substantially uniform momentum across outer cells and center cells.
It is further desirable to provide a method for reducing burner flame deflection and resulting impingement of flames on process tubes in a furnace.